From my discussion at p.g

The attached image illustrates the differenc in light falloff functions.  I have
used a function pigment to illustrate the colour rather than light, but the idea
is the same.

In the image, the red circle represents x=1.
On the right side is the falloff.  The top is the falloff function used by POV,
the bottom is the pure inverse square function.  On the left side I have
illustrated the diffence between the two (subtraction of the formula results).
The top half shows the overcalculation, the bottom the undercalculation.  As we
can see the undercalculation occurs below x=1, and the over calculation occurs
above x=1, with a maximum value at ~x=1.55.

-tgq

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